Due to its superior strength, cast iron, particularly in a pearlitic malleable form, has become an accepted material for pistons in heavy-duty internal combustion engines. However, iron pistons running in iron cylinder bores require protective coatings which provide resistance to wear, seizure, and corrosion, even in the lowest power density engines. In such engines, iron phosphate coatings have been used. To provide greater protection, tin plating has replaced iron phosphate as the iron piston coating of choice, particularly in higher power density engines. As the demand for increased power density continues, even greater protection is required to prevent skirt scuffing, galling, and seizing.
Skirt scuffing is characterized by a loss of tin plating from the skirt and burnt or galled iron evidenced on the skirt and the cylinder bore liner. In general, scuffing begins on the thrust face of the piston. Failures tend to occur at high fuel rates, during run-in. However, problems with skirt scuffing generally are not manifest in those engines which develop low power.
A variety of solutions have been undertaken to address such problems. Some processes call for the spraying of coatings directly onto the iron or onto a protective tin plating. This approach generally reduces productivity, adds cost, and may introduce new problems which may have been solved by the tin plating. Another approach has been to supplement the tin plating with a greater thickness. Such an approach, however, incurs added cost, diminishes productivity, and increases piston-to-bore clearance as the tin plating wears away during the useful life of the piston.
U.K. Patent No. 1 200 453, which was published on Jul. 20, 1970 discloses a high performance diesel engine having cast iron or steel pistons which are coated with an aluminum alloy, the coating being finished to the final required contour of the skirt. The coating is applied as a spray deposit by oxy-acetylene or a plasma-arc spray gun before a subsequent machining step, although the coating may be applied by other methods, such as by casting or plating.
The assignee of the present invention has used a tin babbitt alloy material in its manufacture of remanufactured pistons by flame-spraying a tin babbitt material in wire form over piston skirts. The tin babbitt has a composition of 3.5% copper, 7.5% antimony, with the balance being tin and some trace impurities, including lead, up to about 0.25%. Additionally, the assignee of the present invention practices a process of tin plating piston skirts, in which potassium stannate is used as a coating ingredient. Use of this ingredient is also disclosed in U.S. Pat. No. 4,018,949, which issued on Apr. 19, 1977. That reference discloses a tin deposition applied to a selected zone of an aluminum piston skirt. A warm stream of an aqueous solution containing a protective methyl agent is sprayed onto a selected zone of the piston.
These, among other approaches to the art of providing a wear-resistant coating to piston skirts leave unsolved the question of how to provide an improved coating of iron pistons of internal combustion engines so that they may be better protected against scuffing, galling and seizing at selected zones thereof with high productivity and reliability at a low cost. What is needed is a method of protecting the skirt of a piston against a variety of wear problems when such piston is operated in an engine housing constructed primarily of iron. Such methods should protect selected zones of the piston which are subjected both to wear and a severely corrosive atmosphere. Ideally, the desired method should provide such protection at a reduced cost in comparison to other methods, at equivalent production rates, with existing or similar plating process equipment. Moreover, such pistons should have a limited quantity of a protective coating applied uniformly over such wear surfaces as an outer diameter of the piston skirt (extending from its uppermost to its lowermost region), and at a gudgeon pin/piston interface.